For example, in a data relay apparatus among devices handling data signals, a data signal is reproduced and output based on a clock signal reproduced from an input data signal.
If in this case there is a phase fluctuation (jitter) in the data signal input, then reproduction of the data signal cannot be conducted correctly and a false data signal is output.
In the case where such a device is used in a data transmission system, therefore, it is necessary to previously ascertain the jitter quantity the device can tolerate.
The measurement of the jitter tolerance is conducted by applying phase modulation to a clock signal, which forms a basis of a data signal handled by a measurement subject, with a modulating signal of a sine wave, gradually increasing the amplitude of the modulation signal, and measuring an amplitude of the modulation signal at which it becomes impossible for the measurement subject to handle the data signal accurately. This measurement can be conducted by using, for example, a known pulse pattern generator and a known error measurement device as disclosed in Japanese Patent Application KOKAI Publication No. 4-96533.
In other words, based on the clock signal that is phase-modulated by the modulation signal of the sine wave and consequently that involves jitter depending upon the phase modulation quantity, a data signal having a predetermined pattern is generated based on the clock signal having the jitter by the pulse pattern generator and supplied to the measurement subject.
A data signal output from the measurement subject is compared bit by bit with a data sequence of a predetermined pattern in the error measurement device that is set so as to become the same as the data signal of the predetermined pattern generated by the pulse pattern generator. It is thus determined whether or not an error has occurred.
A limit value of jitter quantity below which an error does not occur is called the jitter tolerance of the measurement subject.
Here, jitter tolerance of the measurement subject is expected to depend on the deviation of the frequency of the clock signal from the frequency of the data signal originally to be handled by the measurement subject and jitter frequency (frequency of the modulation signal).
Therefore, the conventional jitter tolerance measurement apparatus has such a configuration that jitter tolerance measurement can be conducted in two measurement modes.
In a first measurement mode, jitter tolerance is measured while fixing the modulation signal at a standard frequency and varying the frequency of the clock signal, and thereby characteristics of the clock signal frequency versus jitter tolerance are displayed on a display device in a two-dimensional manner.
In a second measurement mode, jitter tolerance is measured while fixing the clock signal at a frequency having no deviation and varying the frequency of the modulation signal, and thereby characteristics of the modulation signal frequency versus jitter tolerance are displayed on the display device in a two-dimensional manner.
However, the above described conventional jitter tolerance measurement apparatus has only the function of independently conducting measurement of jitter tolerance of the measurement subject with respect to the frequency of the clock signal in the first measurement mode and measurement of jitter tolerance of the measurement subject with respect to the frequency of the modulation signal in the second measurement mode and displaying those measurement results individually.
In the conventional jitter tolerance measurement apparatus, therefore, jitter tolerance measurement of the measurement subject must rely on so-called spot measurement in which trial and error of measurement are repeated. This results in a problem that the measurement is extremely inefficient.
Furthermore, even if the measurement of the modulation signal versus jitter tolerance is conducted every clock signal frequency, the conventional jitter tolerance measurement has only the function of displaying the jitter characteristics in a two-dimensional manner. Therefore, jitter characteristics must be displayed individually every clock frequency. This results in a problem that it is impossible to easily ascertain jitter tolerance characteristics and effect an evaluation accurately.
In “1. 2 Test environment”, ITU-T Recommendation 0.171 (04/97), page 13, which is international measurement standards of this kind, there is a stipulation to the effect that “In order to verify the worst-case equipment performance, it may be necessary to stress the equipment under test with multiple change in the test environment.”
In other words, according to the definition in the standard, it is necessary in jitter tolerance measurement to confirm jitter tolerance of the equipment under the severest conditions.
In the conventional jitter tolerance measurement apparatus, however, jitter tolerance measurement is generally conducted in such a state that an offset is not applied to a principal signal, i.e., in a state that is not under the severest condition as defined in the above described standard, for the reason that it takes time or for other reason.